Coagulation Process in Cheese Making
Cheese-making starts with the coagulation of the casein protein in milk. The two main methods by which coagulation can occur are by the action of enzymes or acid. The type of method used determines many of the characteristics of the resulting cheese.
Enzyme Coagulation. The enzyme most commonly used to coagulate milk in cheese-making is rennin, obtained from milk-fed calves, specifically their forth stomach. Rennin, also called chymosin, is sold commercially as rennet. Other sources of available rennin include cows, pigs, plant sources, and genetically engineered bacterial. Alternatively, enzymes may be derived from bacterial starters (Streptococci, Lactobacilli), from certain molds, or from any of a number of other microorganisms. The different enzymes, bacteria, molds, and/or yeasts added during coagulation influence the flavor, texture, and color developed by the cheese during ripening.
The milk is usually heated in large vats at temperatures from 72 to 95F (22 to 35C) to provide an optimal environment for enzymes and bacterial activity, which contributes to the formation of curd. Calcium chloride may be added to speed up coagulation and strengthen the curd’s consistency. Coagulation with enzymes occurs in less than an hour, and creates a tough, rubbery curd. As the curd forms, the whey separates, but most of the milk’s calcium remains in the curd.
Acid Coagulation. There are two methods by which acid may be used to coagulate milk proteins and thus form cheese. The first method is to simply add acid directly to the milk. The second, more complex, method is to inoculate the milk with cultures of bacteria that convert lactose (milk sugar) to lactic acid, which makes the milk medium more acidic. Bacterial cultures have been used for centuries to produce fermented foods, and are carefully selected for their characteristic influences on a cheese’s flavor and texture. It takes from four to sixteen hours to coagulate milk with acid-forming bacteria. About one-fourth to one-half of the calcium in milk is lost in the whey during this process. Curd produced by acid has a soft and spongy texture. This texture is influenced by pH, becoming more solid and compact as the acidity increases. Acid-coagulated cheese is usually not aged because its high acidity inhibits the bacterial and mold growth that characterizes the aging process.
